The present invention is generally related to current to voltage converters, and more specifically to shunt-shunt feedback current to voltage converters.
Current to voltage converters are utilized in a number of applications including electronic devices. One particular type of current to voltage converter is a shunt-shunt feedback current to voltage converter, such as depicted at 10 in FIG. 1. Converter 10 is a typical shunt-shunt feedback current to voltage converter including an operational amplifier 12 having an inverting input, a non-inverting input, and an output vout. Converter 10 is also seen to be coupled to a load ZL, and having a feedback loop comprising of a feedback resistor Rf. As depicted, an input current 14 is coupled to the inverting input of the amplifier 12 and has a value iin.
A typical amplifier 12 has a voltage gain represented as av, an input impedance Zi, and an output impedance Zo. The feedback loop gain can be represented by:   T  =                              R          f                ⁢                  Z          L                                                  R            f                    ⁢                      Z            L                          +                              Z            o                    ⁢                      R            f                          +                              Z            o                    ⁢                      Z            L                                ⁢          a      v        ⁢                  Z        i                              Z          i                +                  R          f                    
As can be appreciated, when input impedance Zi and the load ZL are capacitive, then the loop gain at DC can be reduced to the equation:       T          D      ⁢              xe2x80x83            ⁢      C        =                    R        f                              R          f                +                  Z          o                      ⁢          a      v      
Thus, when the value of feedback resistor Rf is small compared to the output impendence Zo, the feedback resistor Rf""s loading effect decreases loop gain. To maintain the loop gain, the amplifier can have an output stage to achieve a small output impendence Zo. However, the inclusion of such a conventional output stage consumes considerable power.
There is desired an improved shunt-shunt feedback current to voltage converter that reduces or eliminates the loading of the amplifier by the feedback loop resistor Rf.
The present invention achieves technical advantages as a shunt-shunt feedback current to voltage converter including a compensating current provided at the output of the amplifier that mirrors the current at the input of the amplifier such that the amplifier does not provide current to the feedback loop, and hence, the amplifier will not see the feedback Rf as a load. Advantageously, the current provided at the amplifier output mirrors the current at the input such that the amplifier does not provide any current to the feedback loop.
In one preferred embodiment, a current steering digital-to-analog converter (DAC) is utilized that has complementary outputs, these outputs comprising a current source and a current sink of equal values. When the output of the amplifier is positive with respect to common mode, the DAC provides an output sink current to the amplifier input and steers a DAC output source current to the amplifier output. Conversely, when the output of the amplifier is negative with respect to common mode, the DAC provides an output source current to the amplifier input, and steers an output sink current to the amplifier output. Thus, depending on whether the output of the amplifier is positive or negative, the DAC provides complementary outputs to the amplifier input and output such that the amplifier never provides current to the feedback loop and thus does not see the feedback loop as a load which would otherwise reduce the feedback loop gain.
Preferably, a series of polarity control switches are utilized to responsively interface the currents between the DAC and the amplifier input and output. These switches responsively control the current-flow direction to the amplifier input and output as a function of whether the output is positive or negative with respect to common mode.